This invention relates to improvements in a discharge tube firing device using a semiconductor switch as a starter for a discharge tube such as a fluorescent lamp.
There have been already proposed various starters using the semiconductor element for starting the discharge tube, for example, the fluorescent lamp connected across an associated AC source through an inductive stabilizer. One known starter of the type referred to has comprised a nonlinear dielectric element and a reverse blocking triode thyristor each connected across the fluorescent lamp. The thyristor is turned on in a positive half cycle of a source voltage to permit the fluorescent lamp to be pre-heated and off in the next negative half cycle thereof adjacent at the negative peak source voltage. At that time the nonlinear dielectric element is charged with a negative pulsed high voltage due to the inductive stabilizer. The pulsed high voltage is also applied across the fluorescent lamp. The process as described above is repeated until the fluorescent lamp is fired.
The starter as described above is advantageous in that it is cheap and good in starting characteristics but disadvantageous in that, when the fluorescent lamp is re-fired to suddenly increase in voltage thereacross, a charging current from an associated electric source flows into the nonlinear dielectric element and that, after an electric discharge across the fluorescent lamp, a discharging current from that element flows into the fluorescent lamp to increase an electric power consumed by the fluorescent lamp. The charging and discharging currents have caused an electrostrictive vibration of the nonlinear dielectric element and therefore vibrational noise.
In order to eliminate those disadvantages, it has been previously proposed to connect a bidirectional triode thyristor substituted for the reverse blocking triode thyristor, in series to a semiconductor diode across the fluorescent lamp and further connect the nonlinear dielectric element across the semiconductor diode and also in series to a discharging circuit across the fluorescent lamp. The discharging circuit includes a semiconductor diode and a resistor connected in series to each other. The discharging current flows through the discharging network but not through the fluorescent lamp. After the fluorescent lamp has electrically discharged, a voltage thereacross is applied to the bidirectional triode thyristor through the diode and the nonlinear dielectric element has a substantially null voltage applied thereacross resulting in the elimination of vibrational noise.
However the semiconductor diode is connected across the nonlinear dielectric element to prevent a positive voltage from being applied across the latter resulting in the negative pulsed voltage much decreasing in amplitude. This means that it is difficult to start the fluorescent lamp.
Also there has been proposed to connect serially a diode thyristor to the semiconductor diode across the nonlinear dielectric element in order to avoid the disadvantage that the nonlinear dielectric element is not at all applied with the positive voltage. In this measure, a voltage approximating the source voltage is applied across the diode thyristor after the turn-on of the bidirectional triode thyristor and therefore across the nonlinear dielectric element. Thus a positive voltage can be applied across that element.
However when the diode thyristor is turned on with the voltage approximating the source voltage, the fluorescent lamp is initiated to be pre-heated resulting in the insufficient pre-heating of the fluorescent lamp. Therefore the fluorescent lamp has been difficult to be fired.
Accordingly it is an object of the present invention to provide a new and improved discharge tube firing device free from all the disadvantages of the prior art practice as described above.